The invention is related to the field of electrostatography and more specifically to liquid developing systems employed therein.
The formation and development of images on the surface of photoconductor material by electrostatic means is well known. The basic xerographic process as taught by C. F. Carlson in U.S. Pat. No. 2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic charge pattern image by depositing on the image a finely divided marking material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge thereby forming a toner image corresponding to the electrostatic charge pattern. The powder image may then be transferred to a support surface such as paper and permanently affixed to the photoconductive layer if elimination of the powder transfer step is desired. In addition instead of forming a charge pattern by uniformly charging a photoconductor followed by image-wise light exposure, a charge pattern may be formed by directly charging the layer in image configuration. Other methods are known for applying marking particles to the imaging surface. Included within this group are the "cascade" development technique disclosed by E. N. Wise in U.S. Pat. No. 2,618,552; the powder cloud development technique disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776; and the magnetic brush process disclosed, for example, in U.S. Pat. No. 2,874,063.
An additional dry development system involves developing a charge pattern with a powdered developer material, the powder having been uniformly applied to the surface of a powder applicator. The charge pattern is brought close enough to the developer powder applicator so that the developer powder is pulled from the powder applicator to the charge bearing image in image configuration. The charge pattern and powder applicator may desirably be brought in contact including contact under pressure to effect development. The powder applicator may be either smooth surfaced or roughened so that the developer powder is carried in the depressed portions of the patterned surface. Exemplary of this system are the techniques disclosed by H. G. Greig in U.S. Pat. No. 2,811,465.
Development of a charge pattern image may also be achieved with liquid rather than dry developer materials. In conventional liquid development more commonly referred to as electrophoretic development an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with a charged image pattern, the suspended particles migrate towards the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration. Electrophoretic development of a charge pattern may, for example, be obtained by pouring the developer over the image surface, by immersing the imaging surface in a pool of the developer, or by presenting the liquid developer on a roller and moving the roller against the imaging surface. The liquid development technique has been shown to provide developed images of excellent quality and to provide particular advantages over other development methods in offering a development method which lends itself to use in compact reproduction machines.
An additional liquid technique for developing electrostatic charge patterns is the liquid development process disclosed by R. W. Gundlach in U.S. Pat. No. 3,084,043. In this method, a charge pattern is developed or made visible by presenting to the imaging surface a liquid developer on the surface of a developer dispensing member having a plurality of raised portions defining a substantially regular patterned surface and a plurality of portions depressed below the raised portions. The depressed portions contain a liquid developer which is maintained out of contact with the electrostatographic imaging surface. When the raised areas of the developer applicator are brought into contact with the imaging surface bearing a charge pattern, the developer creeps up the sides of raised portions in contact only with the charged area of the imaging surface, and is deposited thereon.
This technique is to be distinguished from conventional liquid development wherein there is an electrophoretic movement of charged particles suspended in a liquid carrier vehicle to the charged portion of the image bearing surface while the liquid substantially remains on the applicator surface and serves only as a carrier medium. In the liquid development method described by R. W. Gundlach in U.S. Pat. No. 3,084,043 the liquid phase actively takes part in the development of the image since the entire liquid developer is attracted to the charged portions of the image bearing surface. Furthermore, in the liquid development method described by R. W. Gundlach, unlike conventional liquid development, the developer liquid contacts only the charged portions of the image bearing surface.
A further liquid development technique is that referred to as "wetting development" or selective wetting described in U.S. Pat. No. 3,285,741. In this technique an aqueous developer uniformly and continuously contacts the entire imaging surface and, due to the selected wetting and electrical properties of the developer, substantially only the charged areas of the normally hydrophobic imaging surface are wetted by the developer. The developer should be relatively conductive having a resistivity generally from about 10.sup.5 to 10.sup.10 ohm cm and have wetting properties such that the wetting angle measured when the developer is placed on the imaging surface is smaller than 90.degree. in the charged areas and greater than 90.degree. in the uncharged areas.
In a compact electrostatographic copying device employing the development techniques disclosed by R. W. Gundlach in U.S. Pat. No. 3,084,043 or in U.S. Pat. No. 2,811,465 by H. G. Grieg the imaging surface and the liquid developer applicator are desirably small diameter cylinders or the like, to facilitate the cooperative movement of the surfaces in contact during development in a confined space. Such moving contact between the imaging surface and the applicator resulting in the transfer of liquid developer from the applicator to the photoreceptor occurs at development speeds ranging generally from about two to about 80 inches per second.
It has been proposed that one of the cooperating surfaces (either the photoreceptor or the applicator) be deformable, having a hardness of from about 30.degree. to about 90.degree. (shore A durometer) while retaining the functional integrity of its operative surface. The use of a deformable surface when at least one of such surfaces is arcuate, provides substantially uniform contact and a substantially uniform nip width between the surfaces.
Although capable of making satisfactory copies, it has been found that in such an arrangement of rigid and deformable surfaces, especially whenever the resilient surface is an imaging surface and the rigid surface is a developer applicator of a narrower width, the edges of the rigid surface are capable of damaging the resilient surface by for example cracking, chipping and scratching. This damage is especially noticeable wherever the pattern of recesses in a rigid applicator coincides with the edge of the applicator to form a file-like profile. Such damage may occur soon after the rigid and resilient surfaces come into contact, and the damage may increase in severity as the surfaces continue to move in contact. Long term moving contact, particularly at high speeds, may result in spreading of the damage across the face of the resilient surface. Such damage may cause unsatisfactory copies to be developed. The areas of a resilient imaging surface which are damaged by chipping, cracking or scraping will fail to support a charge pattern which can be developed. Such damaged areas may also collect unwanted developer which distracts from the image sought to be developed.
It is, therefore, an object of this invention to provide a liquid developing system devoid of the above noted deficiencies.
Another object of this invention is to provide a novel liquid development system.
It is an object of the invention to provide a novel apparatus for reducing the damage done to a resilient roller by a rigid roller.
These and other objects are accomplished according to the invention by providing a rigid member which comprises a rigid surface adapted to move in contacting cooperation with a resilient surface, at least one of said surfaces being arcuate, said rigid surface having at least one end a semi-rigid end extender fitted in such a way that the surface of the end extender lies substantially in the same plane as the rigid surface.